An improved model of the cross-axis synchronous flow-through coil planet centrifuge previously described (Z01 HL 01462-06 LBC) has been tested for protein separations, in both analytical and preparative scale, using a two-phase polymer system composed of polyethylene glycol 1000 (PEG) and dibasic potassium phosphate each at 12.5% (w/w). A mixture of four stable proteins including cytochrome c, myoglobin, ovalbumin and hemoglobin were resolved in both columns at efficiencies ranging from 260 to 670 theoretical plates (TP). Further studies suggested that this relatively low efficiency of the protein separation may be due to the size and/or heterogeneity of the protein molecules rather than high viscosity of the polymer phase system. The present system has been applied to fractionation of E. coli proteins with promising results.